Viewing traffic flow as a complex system, controllers use mathematical and computer models and simulations in an attempt to decrease congestion. Some computer-based traffic models simulate virtual vehicles that motor by the hundreds or thousands through an artificial highway network. The simulations emerge from a more general category of computer models known as agent-based systems, or cellular automata. Others offer a more top-down, conceptual approach to traffic that relies on equation-based models. Spontaneous transformations of traffic, as seen in some of these models, indicate previously unrecognized complexity in highway flow; understanding it requires the concepts and equations
of a relatively new branch of mathematics known as nonlinear dynamics. Traffic engineers are disturbed, however, by the physicists' notion that traffic flow can spontaneously break down into a slow-moving or stopped state...